Ferroli optimax he 38 c Big Problems Please Help

Wathching the tv and i could hear a quiet thumping noise. i traced the noise back to the combi boiler. the display is blank. but on the time clock it is showing a fault with "no cn". the quiet thumping noise is coming from within the boiler unit. does any one know of this fault and how to rectify it as it would be greatly appreciated.P.s i have no hot water. and i have checked electrical supply and water pressure.

The program on BBC 1 and HD from the rememberance celebrations has a ( quite loud ) thumping as I write! Its the drums from the military bands! They have to play loudly because many of the war veterans are quite deaf.